One of the primary steps in the fabrication of modem semiconductor devices is the formation of a film on a semiconductor substrate. This deposition may take place by a number of different processes, including chemical-vapor-deposition (“CVD”) processes in which the film is formed through a chemical reaction of gases. Conventional thermal CVD processes supply reactive gases to the surface of the substrate where heat-induced chemical reactions take place to produce the desired film. Plasma-enhanced CVD (“PECVD”) techniques instead promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, thus lowering the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive.
The deposition of the film may be followed with a polishing step to eliminate irregularities in the surface of the film that may result from the deposition process. One such polishing step may use chemical mechanical polishing (“CMP”) in which a mechanical buffing process is combined with a chemical removal process. The polishing step may have specific uniformity characteristics, depending on the type of technique used and perhaps also depending on the way that technique is implemented. Consequently, the deposited and polished film may include undesirable uniformity irregularities that interfere with further fabrication of the semiconductor device, and perhaps also with the operation of the completed semiconductor device.
Such a combination of the deposition and polishing processes is illustrated in FIG. 1. The process begins at the top of the figure with a substrate 100 over which a film 104 is deposited to produce the intermediate structure 110 shown in the middle of the figure. This film 104 is shown as substantially planar after the deposition process, but a subsequent CMP process performed with a tool that etches the periphery of the film faster than the center results in the final structure 112 shown at the bottom of the figure. Because of the combination of the deposition and polishing processing, the final film 104′ is not planar.
There is, therefore, a general need in the art for a technique that permits forming a substantially planar film that accounts for uniformity irregularities in the polishing process.